物理与天文2018arxiv.org
消解费米悖论
Dissolving the Fermi Paradox
进阶 · 约 35 分钟读完前置知识：德雷克方程、概率分布与蒙特卡洛、天体生物学基础
导读
这篇论文重新审视费米悖论：作者认为，以往用德雷克方程的点估计讨论“外星文明应该很多”并不恰当，应把每个参数都当作分布来处理，并考虑我们对它们的不确定性。把这种不确定性纳入计算后，即便存在费米观测（我们没看到任何外星文明），我们孤独存在于银河系乃至可观测宇宙的概率仍然相当高（论文给出的区间约为 53%–99.6% 和 39%–85%）。
HN 关注点：这究竟算不算“解决”了费米悖论，以及点估计与概率分布之争是否抓住了问题关键。
HN 评论精选
这几条评论里：支持 1 质疑 2 补充 1
补充论文真正的贡献在于把问题从“期望文明数”换成“看到至少一个文明的概率”，而非强调点估计。
这篇论文看似溶解了费米悖论。但我认为它的关键洞见并不是作者所说的“要考虑不确定性而非点估计”，而是意识到德雷克方程和费米观测其实并不冲突，因为它们回答的是不同问题。点估计之所以出现，是因为德雷克方程在假设各因素不相关的前提下给出我们“期望”探测到的文明数；在这个目的下，用点估计完全有效。真正的问题是：就费米观测而言，期望值根本是错的计算量——真正该问的不是期望有多少文明，而是我们“看到多于零个文明”的概率有多大。
Sniffnoy查看英文原文
I'm going to repost the same comment I made on Reddit: This is quite interesting. It certainly sounds like this does dissolve the Fermi paradox, as they say. However, I think the key idea in this paper is actually not what the authors say it is. They say the key idea is taking account of all our uncertainty rather than using point estimates. I think the key idea is actually realizing that the Drake equation and the Fermi observation don't conflict because they're answering different questions. That is to say: Where does this use of point estimates come from? Well, the Drake equation gives (under the assumption that certain things are uncorrelated) the expected number of civilizations we should expect to detect. Here's the thing -- if we grant the uncorrelatedness assumption (as the authors do), the use of point estimates is entirely valid for that purpose; summarizing one's uncertainty into point estimates will not alter the result. The thing is that the authors here have realized, it seems to me, that the expected value is fundamentally the wrong calculation for purposes of considering the Fermi observation. Sure, maybe the expected value is high -- but why would that conflict with our seeing nothing? The right question to ask, in terms of the Fermi observation, is not, what is the expected number of civilizations we would see, but rather, what is the probability we would see any number more than zero? They then note that -- taking into account all our uncertainty, as they say -- while the expected number may be high, this probability is actually quite low, and therefore does not conflict with the Fermi observation. But to my mind the key idea here isn't taking into account all our uncertainty, but asking about P(N>0) rather than E(N) in the first place, realizing that it's really P(N>0) and not E(N) that's the relevant question. It's only that switch from E(N) to P(N>0) that necessitates the taking into account of all our uncertainty, after all! [Note afterward: Over on Reddit, hxka points out that that should be P(N>1), not P(N>0). Or really it should be P(N>1|N>0)...]
质疑以地下海洋和彗星资源为视角，质疑“宜居带岩石行星”这一默认假设。
生态系统受限于水的可获得性，而不是阳光。而且宇宙中更多“宜居”地点是冰线之外的的地下海洋（这是默认状态），而不是极少数被精细调参才有水的岩石世界——仅我们太阳系就有 10–20 个拥有地下海洋的天体。一个星际文明为什么要关心冰线内那些干燥的世界？《洛杉矶之战》最荒谬之处不在于外星人来洛杉矶抢水，而在于它们来一颗只有表面薄薄一层水的行星，而不是去找那些一半质量是水的矮行星、小行星或彗星。如果“黑暗中的文明”掌握氘聚变，它们恐怕没什么理由去碰恒星附近那点可怜资源。
PaulHoule查看英文原文
Ecosystems are limited by the availability of water, not sunlight. Also there are more "habitable" places in the universe which are underground oceans (the default condition outside the frost line) as opposed to a tiny fraction of rock worlds which have to parametrically fine tuned to have water -- ex. there are 10-20 worlds that have underground oceans in our own solar system. Why would a spacefaring civilization care about dry worlds inside the frost line? The most absurd thing about "Battle: Los Angeles" is not that aliens would come to Los Angeles (as opposed to Detroit) to steal water, but that they would come to a planet which has a thin layer on the surface as opposed to being just a run-of-the-mill dwarf planet or asteroid or comet or whatever you call it that is closer to 50% water. If civilizations of the dark had deuterium fusion they might see very little reason to mess around with the puny resources to be found close to a star. It's entirely plausible that a generation "starship" could live off the land and hop from comet to comet to the next star in 10,000 years but probably the crew would lose interest long before the first millenium.
质疑德雷克方程漏掉了许多已知的关键因子（大卫星、超新星），悖论感来自模型不完整。
我很难把它称为悖论，因为在我看来没有什么需要解释的。只要把已知的重要修正因子算进去，再加上还有我们不知道的修正因子就够了。比如传统德雷克方程没把“能在演化时间尺度上稳定自转轴的大卫星”当作主要因子，而这似乎是必需的、且相当稀有。它也没把“生命在演化时间尺度上未被超新星抹掉”算进去——有估计认为这类事件平均每 1500 万年发生一次，而从寒武纪爆发至今约 5.4 亿年，躲过它的概率大约是七十亿分之一。这会给德雷克方程带来多大改变？
btilly查看英文原文
I have trouble calling it a paradox, because there is nothing that needs explaining in my view. Just throw in the fact that known fudge factors are important, and throw in the fact that there are more fudge factors that we don't know. For example the traditional Drake equation doesn't include as a major factor "large moon that stabilizes the axial tilt over evolutionary time". And yet that seems to be needed and at least somewhat rare. The traditional Drake equation doesn't include as a major factor "life not wiped out by a supernova over evolutionary time". And yet per http://earthsky.org/astronomy-essentials/supernove-distance there are estimates that this might happen on average once per 15 million years. From the Cambrian explosion to the present is around 540 million years. The odds of that forbearance is on the order of 7 billion to 1 against. How much does THAT change the Drake equation?
支持把大过滤器置于过去，并由此推论人类很可能是唯一的，不必恐慌。
这篇论文基本上是在说：德雷克方程只是猜测，既然要猜，不如用概率模型来猜。他们的模型把大过滤器放在了我们身后。这也符合直觉：早期的大过滤器只需要阻止生命出现，而晚期的大过滤器要对付的是生命最擅长的事——存活与扩散（“生命总会找到出路”）。具体来说，早期过滤器需要影响的变量更少。正如论文所说，我们很可能就是孤独的，但没什么好恐慌的。如果你相信 AGI 和奇点是智能生命的必然结果，那“我们是第一批、最后一批也是唯一一批”的理由就更充分了：无论大过滤器多强，超级智能都不太可能栽在它手里。
wildermuthn查看英文原文
The paper basically says that the Drake equation is just a guess, and if we’re going to guess, we might as well do so using probabilistic models. Their model puts the Great Filter behind us. This makes intuitive sense too — an early Great Filter simply has to stop life from occurring, while a late Great Filter has to deal with the complexity of preventing life from doing what it does best (surviving and spreading in a “Life finds a way” style). Concretely, an early Great Filter has fewer variables it must affect than a late Great Filter. Like the paper says, we are probably alone, but there’s little reason to be alarmed about it. And if you happen to believe that AGI and a Singularity are the inevitable outcome of intelligent life, the case is even stronger that we are the first, last, and only. However powerful the Great Filter might be, it’s unlikely that a super-intelligence would succumb to it.
历次讨论
- 2018Hacker News 讨论帖113 分 · 126 评
- 2021Hacker News 讨论帖50 分 · 71 评
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